Optical intersatellite link terminals are utilized to communicate between two satellites. One of the principal technical challenges in designing communication networks involving these terminals is to isolate the receiver channel within any terminal from back-scatter or any other spurious radiation that may be produced by the transmitted beam originating within the same optical terminal. For typical intersatellite ranges and acceptable transmitter/receiver aperture sizes, the isolation requirement is often greater than 90 dB, i.e. any spurious signal generated by the transmitter beam which can possibly enter the receiver channel must be less than the transmitter beam itself by at least 90 dB.
In communications networks incorporating optical intersatellite links (OISLs), one common isolation approach which achieves acceptable size and weight while also producing the required isolation between transmitted and received beams is to operate the beams at different wavelengths. A dual wavelength OISL network requires (at least) two types of terminals which are characterized by their operating wavelengths. For example, terminals of type A transmit radiation at a first wavelength and receive radiation at a second wavelength, while terminals of type B transmit at the second wavelength and receive at the first wavelength. A successful communication link requires that an A terminal communicate with a B terminal. The operating wavelengths are typically separated by about 5 to 10 nanometers (nm) which is large enough that available receiver bandpass filters can adequately reject spurious transmitter radiation and meet the isolation requirement. This approach offers various advantages, including the fact that the transmit and receive functions can share a common telescope and pointing optics to reduce weight and cost. To separate the transmitted beam from the incoming received beam, a dichroic beamsplitter is used which reflects the first wavelength while transmitting the second wavelength.
A second known isolation method employs polarization-based switching techniques to isolate the transmitted and received beams which operate at the same wavelength but orthogonal polarizations. This approach provides isolation of only about 30 dB to 40 dB depending upon the particular implementation and is therefore not appropriate for many OISL applications which have more stringent isolation requirements.
While physical isolation of the optical paths provides acceptable isolation, this third approach requires separate telescopes and pointing optics and therefore results in greater weight and cost compared to the approaches described above. Likewise, temporal isolation, i.e. transmitting and receiving at different times, represents a fourth approach that may be used to provide sufficient isolation but imposes severe constraints on the communication format that can be used. Furthermore, temporal isolation requires the system to adapt to changes in intersatellite range and is therefore undesirable.
Designers of space-based communications networks recognize that some fraction of the OISL terminals will become inoperable during the desired system lifetime. To compensate, sufficient redundancy should be provided so that the overall system can function acceptably despite the loss of a considerable fraction of the terminals. A typical system design may specify 50% more terminals per satellite than is required to maintain minimum acceptable performance. An implicit assumption when this reliability approach is applied in conjunction with dual-wavelength isolation is that any given satellite will always have enough terminals of the proper type (A or B) to maintain its links with complementing terminals of the opposite type on one or more other satellites. To reduce cost while providing acceptable system redundancy, the terminals of the communication system should be reconfigurable such that an A terminal can convert to a B terminal (and vice versa) upon receipt of an appropriate command from the network controller.
One approach to providing this reconfigurability is to mechanically exchange a dichroic beamsplitter so that the appropriate wavelength for the A or B terminal is directed to the receiver channel and from the transmitter. However, this approach requires complex and costly mechanical and electronic componentry to achieve the precise alignment necessary for the repeated switches between type A and B terminals throughout the life of the satellite network.